1. Field of the Invention
The present invention relates to a method of measuring the distance between a fixed point and an object by optical pulse time delay measurement, including the steps of:
a) transmitting a light pulse from the fixed point at a selected instant of transmission;
b) periodically scanning (sampling) of light intensity received at the fixed point and continuously storing, as a set of received scanned (sampled) signal values, the scanned (sampled) values at the scanning (sampling) rate during a predetermined measuring time window embracing the instant of reception of the light pulse reflected from the object;
c) repeating steps a) and b) N number of times in order to obtain N number of sets of received signal values, wherein N is an integer equal to or greater than 1;
d) summing the individual stored sets of received scanned (sampled) signal values, set by set, to a summed scanned (sampled value set during the calculating time window following the N measuring time windows.
By summing the N received signal scanned value sets, the signal to noise ratio of the received signal is improved proportionally to the root of N. The echo (the reflection) of the light pulse is sought in the sum signal, and the distance may be derived on the basis of the known equation of distance equaling half the product of scanned value number of the echo, pulse rate and speed of light.
2. The Prior Art
The described principle of improving the signal to noise ratio has been known for a long time and has been described in Optical and Quantum Electronics, Vol. 7, No. 3, 1975, pp. 179-185. The disadvantage of the known method is that for a ten-fold increase of the signal to noise ratio it is necessary to store 100 sets of received signal scanning values of, for instance, 1,000 scanning values which thus requires 100,000 memory locations.
On the other hand, a second group of optical distance measuring methods is known from EP 0,312,524, in which real time calculation of the sum scanning value is performed during scanning. Each scanned value is continually added at the scanning frequency rate to corresponding scanning values which are present already. While the requisite storage requires (in the example referred to) only 1,000 cell, it is necessary to provide considerable computing power, since the time available for adding two values at a conventional scanning frequency of, e.g., 20 MHZ amounts to only a few nanoseconds. This requires either powerful application specific integrated circuits (ASICs) or digital signal processors which makes the construction of corresponding circuits as expensive as the large storage components of the first group of methods.
Moreover, in praxi it is not possible to gain time by real time processing since the pulse repetition rate of conventional laser diodes for thermal reasons as well as for reasons of eye safety is limited to a range of about 10 kHz. The time between termination of a scanning operation (at a distance of 1,000 m, for instance, this amounts to about 6.6 xcexcs following the transmission of a laser pulse) and transmission of the next laser pulse remains unused in practice.
It is, therefore, an object of the invention to provide a method of distance measuring, based upon the optical impulse time delay method, of improved signal to noise ratio as a result of received signal summation, which may be realized by circuit means which are simpler and more cost-efficient than those of known methods, and which, considering the operational limitations of conventional light pulse sources, allows to perform measurements in a short time period.
The object is accomplished by a method of the kind referred to supra which includes the further steps of:
e) repeating steps a) through d) in order to obtain a further summed scanning value set, whereby during or following step d) the further summed scanned value set is added, scanned value set by scanned value set, to the present summed scanned value set to actualize the latter, and whereby an equalizing portion of a summed scanned value set is optionally subtracted therefrom before and/or after the mentioned addition;
f) searching within the summed scanned value set for significant scanning values which satisfy predetermined threshold values;
g) repeating steps e) and f) until significant scanning values have been detected; and
h) determining the looked-for distance from the position of the detected Significant scanning values in the summed scanned value set.
In this manner, time separated pulse trains of successive individual pulses are transmitted. The received signal scanned value sets of the individual pulses are merely stored during scanning and are summed only after reception of a complete pulse train. However, the summed scanned value sets of successive pulse trains are summed during the pauses between the impulse trains.
The invention is based upon the recognition that arrangements of conventional light pulse sources, such as laser diodes, may be operated to provide pulse trains, with sufficient time being provided between pulse trains to perform xe2x80x9cslowxe2x80x9d calculations so that the necessary circuits can be provided as simple and cost-efficient micro-processors. Only a few received signal sets have to be incorporated in the individual pulse trains so that no large storage devices will be required. Moreover, the reduction of the summed scanned value sets by their equalizing portion following each pulse train reduces the bit width of the scanned value sets to be stored so that the storage need be of small depth only.
Based on actual trails to be described hereafter it has been found that a particularly advantageous compromise may be achieved between the mentioned opposite requirements if in accordance with a preferred embodiment of the invention the calculating time window is 4 to 16 times, preferably from about 8 to 10 times, the N measuring time windows.
Increased resolution may be obtained if preferably in step h) the center of gravity of the significant scanning values is used as the position.
Furthermore, it is particularly advantageous if in case of several spaced groups of significant scanning values a first group which is markedly wider than the next group is ignored in steps f) to h). In that manner the target echo may be distinguished from an echo resulting from visual interference as may occur at close range and which typically is wider than the target echo.
In order to prevent a background from distorting the measurement when measuring an object in front of such background the last group or groups are preferably ignored in steps f) through h) when there are several groups of significant scanning values spaced from each other.